The ALMA Back End systems deliver signals generated by Front End units installed in each antenna to the Correlator installed in the AOS Technical Building. Signal processing and data transfer is schematically shown in the picture below. Analog data, produced by the Front End electronics, is processed and digitized before entering into the data encoder, followed by the optical transmitter units and multiplexers. All these elements are installed in the receiver cabins of each antenna. Optical signals are then transmitted by fibers to the AOS Technical Building. The total distance is, in one antenna configuration, about 15 kilometers. At the Technical Building the incoming optical signals are de-multiplexed and de-formatted before entering the Correlator.

Schematic of ALMA Signal Processing and Data Transfer from the Front End to the Correlator.

The ALMA main array Correlator

The ALMA main array Correlator, to be installed in the AOS Technical Building, is the last component in the receiving end of the data transmission. It is a very large data processing system, composed of four quadrants, each of which can process data coming from up to 504 pairs of antennas. The complete correlator will have 2912 printed circuit boards, 5200 interface cables, and more than 20 million solder points. Integral parts of the Correlator are Tunable Filter Bank (TFB) cards. The layout is such that four TFB cards are needed for the data coming from a single antenna. The TFB cards have been developed and optimized by the University of Bordeaux over the last few years.

The Atacama Compact Array (ACA) Correlator

The ACA Correlator is designed to process the signals detected by the Atacama Compact Array
(ACA). This correlator consists of 52 modules connected with each other through optical-fiber cables. All the modules
are installed in 8 racks in the AOS Technical Building. The power spectra issued from the correlation are
transferred to the ACA data processing computers.